When you write, your reader has the time to reflect on what they just read before reading further. Since your reader can set aside the time to process what you’ve written, you can write at length and in detail about the topic at hand.
When you speak, this isn’t the case. Discussions progress fast and the recipient of your communication doesn’t have the time to perform deep reflection on what was just said before you either jump on to a related point, or they jump in to fill in an awkward silence.
As a result, being an effective speaker requires a different approach than being an effective writer. Specifically, effective speaking requires speaking in small bite sizes that your recipient can absorb in real-time, at a level of detail that is sufficient to communicate the core of your message without losing your recipient’s attention in an attempt to understand each of the message’s underlying reasons.